In the prior art it has been proposed to test materials and to excite solutions and slurries with the use of ultrasonic energy. Slurries are defined as one phase (liquid or solid) dispersed in another liquid phase such as oil droplets in water or coal particles in methanol. In essence, whether it is in connection with a solid material, a liquid solution, or slurry, it is common to send ultrasonic pulses from a transducer, that usually takes the form of a crystal such as a barium titanate, lead zirconate titanate, lithium niobate etc., into the object being tested. In some instances, the same crystal acts as a receiver and detects the acoustic echoes that might reverberate from the test material. In some cases, moving fluids have been studied using these methods. However, practical measurements using these techniques have been limited to acoustic attenuation and velocity.
In 1933 P. Debye, (Journal Chemical Physics, Vol. I, Pg. 13, 1933), predicted that when sound is propagated in an ionic solution, a momentary charge separation occurs because of the different ionic mobilities. This predicted effect occurs if cations and anions of an electrolyte have different effective masses and frictional co-efficients. This momentary charge separation, where one region will be charged positively with respect to an adjacent negatively charged region implies than an instantaneous electrical signal can be measured. Thus, if inert metal probes are placed in two different regions, a potential will be observed with the same frequency as the sound wave. A similar effect arises for colloidal particles and emulsion droplets because of the distortion of their ionic surroundings. This concept was first predicted by A. Rutgers in 1938 (Physica 5:46) and observed further by E. Yeager in 1949 (Journal Chemical Physics, Vol. 17, Page 411). The modern method for measuring the electrical potential in colloidal systems is given by U. Beck et al (TAPPI Vol. 61, Pages 63-65), and a further arrangement for ionic solutions is seen in Borsay et al (Journal of the Acoustical Society of America, Pages, 240-242, Vol. 64, July, 1978). In all of the prior experiments, difficulty has been encountered in the fact that the voltage being measured is quite small, and depends on the sound amplitude at the electrodes and the electrical conductiviy of the liquid. It is desirable therefore to improve upon the system for measuring the electrical potential, predicted by Debye. This potential has been termed "The Ultrasonic Vibrational Potential" (U.V.P.) and is closely related to the concept of zeta potential. We have, therefore, used the inverse effect from the one described by Debye, that is to say the sound is generated by an electric field, in a practical device to study liquids.